Research in Astron. Astrophys. Vol.0 (20xx) No.0, 000–000 Research in http://www.raa-journal.org http://www.iop.org/journals/raa Astronomy and (LATEX: 2020-0222.tex; printed on November 11, 2020; 17:21) Astrophysics Search for the brightest stars in galaxies outside the Local Group ∗ N.A. Tikhonov1, O.A. Galazutdinova1, O.N. Sholukhova1, A. Valcheva2, P.L. Nedialkov2, O.A. Merkulova3 1 Special Astrophysical Observatory, Nizhnij Arkhyz, Karachai-Cherkessian Republic, Russia 369167; [email protected] 2 Department of Astronomy, Sofia University, Sofia, Bulgaria, 1504 Sofia, 15 Tsar Osvoboditel Blvd. 3 Astronomical Institute, St.Petersburg State University, Universitetskii pr. 28, St.Petersburg, 198504 Russia Received 2020 June 13; accepted 2020 November 15 Abstract This paper shows a technique for searching for bright massive stars in galax- ies beyond the Local Group. To search for massive stars, we used the results of stellar photometry of the Hubble Space Telescope images using the DAOPHOT and DOLPHOT packages. The results of such searches are shown on the example of the galaxies DDO 68, M 94 and NGC 1672. In the galaxy DDO 68 the LBV star changes its brightness, and in M 94 massive stars can be identified by the excess in the Hα band. For the galaxy NGC 1672, we measured the distance for the first time by the TRGB method, which made it possible to determine the luminosities of the brightest stars, likely hypergiants, in the young star formation region. So far we have performed stellar photometry of HST images of 320 northern sky galaxies located at a distance below 12 Mpc. This allowed us to iden- tify 53 galaxies with probable hypergiants. Further photometric and spectral observations of these galaxies are planned to search for massive stars. Key words: stars: massive, stars: variables: LBV, galaxies: individual: M 94, DDO 68, NGC 1672 1 INTRODUCTION One of the main tasks of modern astrophysics is to determine the upper limit of the mass of a star. The interest lies in the fact that massive stars evolve quickly and the final stage of their evolution can be a supernova burst, formation of a black hole or a neutron star, or even a merger of such relativistic objects with the release of huge amount of energy. The evolutionary path of a star depends on its initial mass. Models of stellar evolution suggest that the initial masses of stars can reach up to 500 solar masses (Yusof et al. 2013; Spera & Mapelli 2017) and more, but so far, only the stars of much smaller mass have been discovered (Crowther et al. 2010). The question of the fidelity of theoretical models can only be solved by searching for and studying massive stars in galaxies of various masses. In our Galaxy, massive stars are observed in clusters located in the galactic plane, where the extinc- tion of light can be very high (Eikenberry et al. 2004). To increase the breadth of the search and solve the problem with the uncertainty of light extinction, one should go beyond the Galaxy and search for massive stars in other, not too distant galaxies (Humphreys et al. 2017; Sholukhova et al. 2018). Using the Hubble Space Telescope, numerous galaxies outside the Local Group are accessible for the study (up to distances of 15–20 Mpc). ∗Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc. under contract No. NAS5–26555. These observations are associated with proposal 10402,10354,11578,14716,15654. 2 N.A. Tikhonov et al. 2 SEARCH FOR CANDIDATES FOR MASSIVE STARS Spectral observations make it possible to determine the type of star and evaluate its physical parameters, but such observations are not suitable for search tasks because of the large number of candidates for massive stars and high time expenditures to obtain the results. Therefore, to identify possible candidates for massive stars (hypergiants), photometric methods are used, as they are faster and more accessible on all telescopes. It seems that finding massive stars, which are also the brightest stars in galaxies, is not difficult, but this opinion is incorrect. In many low-mass galaxies, there is simply no such stars. The lifetime of these stars is very small (several million years), and the frequency of their appearance in galaxies is small. According to the law of mass distribution of Salpeter, thousands of stars of smaller masses should be born per star with a mass of 100 Solar masses. Even in our giant Galaxy, with numerous of star formation regions, only a few stars with masses of 150—300 Solar masses are known: HD15558A (Garmany & Massey 1981), Eta Carinae (Gull 2015). The second factor that complicates the search is the superposition of stars of our Galaxy on the studied fields. An example is given by the metal-poor dwarf galaxy DDO 68 (Figure 1), the distance to which is 12±0.3 Mpc (Tikhonov et al. 2014). In this galaxy, a bright massive LBV (Luminous Blue Variable) star is known (Pustilnik et al. 2017), which is marked on the image of the galaxy (Figure 1). On the Hertzsprung-Russell diagram (CM-diagram) DDO 68 (Figure 2), this star is very bright (according m to our measurements, MV = −10 . 26). However, this CM-diagram contains another brighter blue star, also visible on the body of the galaxy (Figure 1). As shown by spectral observations with 6-m telescope BTA, this bright blue star has no emission lines, and its radial velocity is low compared to the DDO 68. This indicates that it is not a massive star DDO 68, but belongs to our Galaxy. Isochrones of Bertelli et al. (1994) overploted on the DDO 68 CM-diagram also show that the bright blue star does not belong to the stars of the DDO 68. Thus, the probability of a foreground star falling into the selection of candidates for massive stars is far from zero. A convenient feature for identifying foreground stars Fig. 1: Dwarf metal-poor galaxy DDO 68 in the F606W (V ) filter with the LBV star, marked by a circle and a bright blue field star belonging to our Galaxy. is to compare the positions of bright stars with respect to star formation regions. The probability that a massive star will appear in a star cluster is always higher than the probability of its isolated appearance. A comparison of the positions of two bright blue stars in the DDO 68 galaxy shows that the LBV star is indeed surrounded by a cluster of stars and gas clouds, while the foreground star is completely isolated. However, this feature is not absolutely mandatory, since bright massive stars are known outside the clusters. Search for the brightest stars in galaxies outside the Local Group 3 Fig. 2: CM-diagram of DDO68 stars and isochrones of age from 3 Myr to 100 Myr and metalicity Z = 0.001 from Bertelli et al. (1994). The position of the LBV star and the foreground star is marked. Fig. 3: Images of the galactic region DDO 68 in 2010 and 2017 in the F606W filter, in which the LBV star is marked by a line. A large change in the luminosity of the LBV star over 7 years can be seen. The third factor that impedes the search is the insufficient spatial resolution of the telescopes. Even in our Galaxy, when using ground-based telescopes, it is not possible to separate close groups of massive stars into separate stars. Therefore, when the mass of the entire group of stars is attributed to a single star, errors in mass estimation can occur. The Hubble Space Telescope allows to separate close groups and evaluate the luminosities of individual stars, but it is still not enough for distant galaxies. The unstable state of massive stars often leads to variability of their brightness, which can be used for search of such stars. However, the constancy of the brightness of a star over several years does not mean its stability for a longer time interval. Only long series of accurate photometric measurements can give an answer about brightness variability. Figure 3 shows the images of LBV star in DDO 68 in 2010 and 2017. The pictures clearly show a strong drop in its brightness from V = 20 m. 19 (2010) to m m m V = 23 . 70 (2017) (from MV = −10 . 26 to MV = −6 . 75). Due to the instability of massive stars, gas loss occurs and various shells and clouds form. In the spectra of such stars, a strong Hα hydrogen emission line should be observed. The presence of the emission line greatly simplifies the search of candidates for massive stars. Images of the galaxy in Hα filter, together with images in other filters (F435W, F555W, F606W and F814W) allow to highlight bright blue stars with Hα emission, which are likely to be the sought-after massive stars. But even after such a selection, extraneous objects fall into the list. Young compact star clusters almost do not differ in profile from single stars; they have a strong Hα emission and a small color index, identical to the color index of bright massive stars. High resolution spectral observations help to separate stars from clusters, but this requires additional research. 4 N.A. Tikhonov et al. Fig. 4: Part of the galaxy M 94 in HST image in the F555W filter. The positions of four bright blue stars with increased brightness in the Hα filter are marked. Fig. 5: Regions of stars marked in Fig. 4 with numbers. Star N4 is shown off-center.